Melamine is commercially produced by heating urea to provide melamine and ammonia and carbon dioxide as by products. The basic reaction is ##STR1## The commercial processes are either high-pressure, non-catalytic or low-pressure and catalytic using a catalyst such as alumina. Conventionally, in low-pressure, catalytic processes the melamine is recovered in an impure form and, subsequently, recrystallized using a chemical treatment to provide melamine which is essentially 100% pure. Similar, chemical treatment and recrystallization steps were used in producing pure melamine with the high-pressure, non-catalytic process.
U.S. Pat. No. 4,565,867 issued Jan. 21, 1986 and assigned to the assignee of the present application, describes a high-pressure process wherein the melamine is recovered at a relatively high purity and used in that form without a crystallization step. The process is highly efficient and provides a low cost melamine. In carrying out the process, urea melt is fed into a scrubber unit at from about 1500 to 2500 psi pressure, preferably from about 1700 to 2200 psi, and at a temperature above the melting point of urea. In the scrubber unit, the liquid urea contacts reaction offgases principally composed of CO.sub.2 and NH.sub.3 and containing melamine. The urea, in molten condition, scrubs the melamine from the offgas. In the scrubbing process, the offgases are cooled from about the temperature of the reactor, i.e., from about 670.degree. to 800.degree. F. to from about 350.degree. to 450.degree. F. The temperature and pressure are interrelated. If the pressure is at the low end of the range, i.e., 1500 to 1700 psi, the minimum temperature of the scrubber will vary from about 350.degree. to 360.degree. F.; whereas if the scrubber is at the high end of the pressure range, i.e., 2000 to 2200 psi, the minimum temperature can be increased to about 360.degree. to 380.degree. F. Below the above minimum temperatures ammonia and CO.sub.2 condense in the bottom of the scrubber and may form carbamate which can be detrimental. As a rule of thumb, the higher the pressure the higher the required minimum temperature. Above about 500.degree. F. the urea may react to form intermediate products. These intermediate products can be detrimental.
A gas separator is provided in the system wherein liquid melamine is separated from offgases, and liquid melamine is collected in the bottom of the separator. The separator is held at a temperature above the melting point of melamine. The gaseous ammonia and carbon dioxide saturated with melamine vapor are removed overhead and fed into the urea scrubber. The liquid melamine is removed from the gas separator on level control and injected into a product cooling unit.
A unique feature of the process described in the '867 patent is the cooling unit wherein liquid melamine recovered from the separator is depressurized and rapidly cooled with a liquid medium which will form a gas at the temperature of the liquid melamine. By utilizing the rapid depressurization and quenching, the liquid melamine is directly converted to a solid powder having a high purity without washing or further purification. As is disclosed in the '876 patent, the liquid quenching agent is a low boiling liquid which gasifies with the gas being readily separated from the solid melamine product. Suitable quenching agents are ammonia, water, or a low boiling alcohol. As further disclosed in the '867 patent, the pressure of the quenching can be atmospheric pressure or a pressure up to about 600 psi. According to the '867 patent, it was preferred to operate at a pressure of about 200 to 400 psi and a temperature of from about 120.degree. to 165.degree. F. In the disclosed process the pressure, as above defined, is the same in the scrubber, reactor, and gas separator. The offgases removed from the gas separator are at the same temperature as the reactor and separator until they reach the scrubber where they are cooled in the process of being scrubbed with the molten urea. The liquid melamine transferred from the gas separator enters the product cooling unit at the same temperature range as the reactor and gas separator.
Although the process of the '867 patent has produced melamine in the range of 96 to 99.5% melamine which contains low levels of melem and melam, the process of the '867 patent in commercial operation has only produced melamine in the range of about 97.5% with the main impurities being melem, melam, urediomelamine and ammeline. Although this product is usable in most melamine markets, it is limited in some because of the impurities. In high pressure melamine technologies other than as described in the '867 patent, the impurities are removed by using a chemical treatment and then a recrystallization process. The impurities are converted to components that can be removed from the product by either filtration or sedimentation or both. This reduces the yield of melamine from urea by removing these impurities and also increases the disposal cost by having to dispose of the filter cake.
There is a need, therefore, to produce a more pure melamine, namely 99+ percent, on a commercial basis without either recrystallizing or having to dispose of the by products.